Experiments were performed at the horizontal shock tube facility at Los Alamos National Laboratory to study the effect of incident shock Mach number (M) on the development of the Richtmyer-Meshkov instability after a shock wave impulsively accelerates a varicose-perturbed, heavy-gas curtain (air-SF6-air). Three cases of incident shock strength were experimentally investigated: M = 1.21, 1.36, and 1.50. The resulting instability and subsequent fluid mixing is measured using simultaneous quantitative Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) for the first time in a Richtmyer-Meshkov Mach number study, while exceptional experimental repeatability allows for isolation of Mach number effects. Investigated are the mechanisms that drive the mixing, at both large and small scales, by examining the time evolution of simultaneous, 2-D density and velocity fields for each Mach number. Several differences in qualitative flow features are identified as a result of Mach number variation, with differences in vortex interaction playing a critical role in the development of the flow field. Several quantities, including mixing layer width, mixing layer area, interface length, vorticity, circulation, velocity fluctuations, instantaneous mixing rate, the density self-correlation parameter, and other measures of turbulence and mixedness are examined as a function of time. These quantities are also examined versus time scaled with the convection velocity of the mixing layer, showing that the rate of change of several of these quantities with the distance the mixing layer travels is independent of Mach number. Results show that higher Mach number yields greater mixing uniformity at a given downstream location, while lower Mach number produces greater amount of mixing between the two gases, suggesting possible implications for optimization in applications with confined geometries.